1. Technical Field of the Invention
The present invention relates to a method and apparatus for measuring the permeability to water of pervious sheets. More specifically, it relates to a method and apparatus for measuring the permeability to water of pervious sheets such as forming fabrics (also referred to as wires) and press felts as used in paper, board and pulp production processes.
2. Prior Art
In paper and board production plants, pervious sheets such as felts and forming fabrics are used for dewatering of aqueous suspensions of the pulp fibers in the production of paper and board. The permeability to water of these sheets is an important factor to the production efficiency as well as to the quality of the product obtained. Methods and apparatuses for evaluating this factor accordingly have been sought.
Some instruments of the prior art make use of the measurement of the flow rate of a stream of air through the pervious sheet used in the paper machine, see e.g. U.S. Pat. Nos. 2,722,837; 3,091,123 and 3,397,319. However, it is highly questionable whether the measurement of the permeability to air could provide an accurate measurement of the felt or forming fabric performance, as this is related not to the permeability to air but to water. Moreover, as taught in the Canadian patent no 1,292,891, this instrumentation of the prior art suffers from a poor reproducibility.
In view of this, instruments for the measurement of the water permeability by use of a flow of water have been developed. The first ones of this type of instruments were of a stationary nature and were used in laboratory measurements, see e.g. the U.S. Pat. Nos. 3,577,767 and 4,385,517. To be able to make a measurement of the permeability to water of a pervious sheet used on a paper machine, one thus had to cut out a sample piece and take it to the laboratory. This obviously excludes the method from being applicable in an on-line mode, such as would be desirable from a production efficiency point of view.
Consequently, in the art a need still existed for a method and apparatus for measuring the permeability to water of a pervious sheet on a paper machine in an on-line mode, to be able to quickly ascertain the performance of the pervious sheet, without having to destroy it such as by cutting out a piece of it. This was the object of the Canadian patent no 1,292,891, wherein a method and an apparatus for measuring the permeability to water of a pervious sheet material are disclosed. The method taught in this patent can be used in a dynamic mode, i.e. on a pervious sheet material, such as a press felt or a forming fabric in rapid movement in the paper machine, and consists of measuring the flow of a stream of water injected at constant pressure through the material. Thus, this document discloses placing a nozzle against one surface of the material, flowing water through said nozzle at a pressure between 2 kPa and 1000 kPa and measuring the rate of flow of the water. A commercially available instrument operating according to this principle is the SCANPRO FELT-PERM.TM., manufactured and sold by Lorentzen & Wettre, of Sweden.
This measuring method as taught in said Canadian patent, while being optimized for measuring the permeability to water of a pervious sheet material such as a press felt, is less adequate for measuring the permeability to water of a pervious sheet material of the type of a forming fabric. Indeed, the properties of a forming fabric differ substantially from those of a press felt. The former is a loose woven gauze of rather coarse plastic threads and very permeable to water, while the latter is a close woven structure of a much lower permeability to water. It thus has been found that the dynamics of the measurement signal is inadequate when using the method based on measuring the flow of water at constant pressure to assess the permeability of a forming fabric, i.e. the flow rate varies only negligibly in response to the variation of permeability. As a result, the method will not give an adequate indication of the degree of wear and clogging of the forming fabric.
Moreover, in the instrumentation according to the Canadian patent, the nozzle ranges from 1 to 4 mm in diameter, and the pressure of injection of the water ranges from 2 to 1000 kPa. While being suitable for measuring the permeability of a press felt, the high pressure, small diameter water jet produced, in the case of a forming fabric will lead to a local cleaning away of fibers caught onto the surface of the fabric and this in turn will result in an uneven dewatering of the pulp on the thus locally cleaned forming fabric.